Nonvolatile memory, which is regarded as a mainstream type of memory in the market, is realized using a technology of changing the threshold voltage of a semiconductor transistor due to electric charges stored inside an insulator film, which is disposed above a channel, as being represented by flash memory or silicon oxide nitride oxide silicon (SONOS) memory. Although miniaturization is unavoidable for the realization of high capacity, it is difficult to miniaturize a single piece of a semiconductor transistor that does not have a charge storage function. Therefore, underway are studies of allowing the transistor to perform only the function of a switch to select a memory cell that reads and writes and separating a memory device like dynamic random access memory (DRAM), such that miniaturization can be carried out for respective devices for the purpose of high capacity.
When continuously miniaturizing data storage mechanism, it is considered to use a resistive change element as a memory device. The resistive change element uses an electronic device that can convert an electrical resistance into binary or more due to any electrical stimulus. In the method of storing charges in a capacitor such as DRAM, it is unavoidable that a signal voltage decreases since the amount of charges that is stored is decreased due to miniaturization. Meanwhile, it is regarded that an electrical resistance generally has a definite value in many cases even though miniaturization is carried out, and that it is advantageous to continue miniaturization if there are a principle and a material by which the value of resistance is changed. The resistive change element operates as a switch that converts between an “on” state, i.e. a low-resistance state, and an “off” state, i.e. a high-resistance state. For example, it is possible in principle to apply the resistive change element to a switch 33 that enables a first wiring 31 and a second wiring 32 to be connected to each other, as shown in FIG. 3, and to a converter for a wiring configuration in a large scale integration (LSI).
A plurality of existing technologies is known as technologies for changing an electrical resistance due to an electrical stimulus. Among them, the technology that is best studied relates to a memory device that changes the crystalline state thereof (crystal phase-amorphous phase) in response to pulse current supplied to chalcogenide semiconductor and uses that there are two or three digits of difference in an electrical resistance of each phase. This is generally referred to as phase change memory. In a structure of metal/metal oxide/metal (hereinafter, referred to as an MIM type), in which a metal oxide is positioned between metals, it is also known that a change in resistance is caused by applying a strong voltage or current. The present invention relates to this MIM type device.
FIG. 4 is a schematic cross-sectional view of an MIM type resistive change element. This is a structure in which a resistive change element film 42 made of a metal oxide is interposed between an upper electrode 41 and a lower electrode 43. For example, according to Non-patent Document 1, a resistive change element in which a nickel oxide (NiO) is used for a resistive change element film 42 was reported. From 1950s to 1960s, studies on a variety of materials that exhibits a phenomenon in which a resistance value is changed by a voltage or current were already reported.
FIG. 5 shows current-voltage characteristics of such an MIM type resistive change element. This device remains in the high-resistance off state or the low-resistance on state in a nonvolatile way when power is turned off and the resistance state can be converted by applying a predetermined voltage/current as required. An example of the current-voltage characteristics of the on and off states is shown in FIG. 5. When a voltage of Vt1 or higher is applied to a device that is in the high-resistance off state, the device is converted into the low-resistance on state, and shows the electrical characteristic of FIG. 5(b). In turn, when a voltage Vt2 or higher is applied to the device that is in the on state of FIG. 5(b), the device is converted to the high-resistance off state and returns to the electrical state of FIG. 5(a). The operation of repeatedly converting between FIG. 5(a) and FIG. 5(b) is possible, and this characteristic can be used for a non-volatile switch or a non-volatile memory cell that serves to convert a circuit.
In an MIM type resistive change element that contains a metal oxide, a current path that is in charge of a low-resistance state is not formed in the entire electrode surface, as shown in FIG. 6. It is characterized in that the current path is defined by a localized current path 44 that has a diameter of about several nanometers, and at most, tens of nanometers. FIG. 7 shows the electrode area dependency of a resistance value in the low-resistance state of a parallel flat MIM type resistive change element, which uses NiO as a material for changing a current path resistance like Non-patent Document 1 above and is positioned between electrodes. FIG. 7 clearly demonstrates that the resistance value in the low-resistance state rarely depends on the electrode area, and that the low-resistance state is managed by the current path that is localized.
In order to enable such a current path, it is necessary to suppress an additional current path the resistance of which is not variable from occurring. Such an additional current path is mainly a byproduct 51, which is attached to a side wall at the time of etching, or a damage 52, which is formed on the side wall in the processing of a device, as shown in FIG. 8. In particular, when a resistive change material that contains a magnetic material such as Ni is used, the vapor pressure of the reaction byproduct is low and thus the probability that reaction byproduct is to be attached increases. When the upper and lower electrodes are short-circuited by the byproduct 51, if the electrical resistance of the byproduct is small, the function as a resistive change element is of course disabled by an excess current path indicated by i1. Even when the resistance is higher than the on state, the resistance of the off state is lowered. In addition, when the damage 52 is introduced, the characteristics of the resistive change element are deteriorated by an excess current path indicated by i2. When the device is used as a switching device, it is required for the high-resistance state of the device to realize a stable high-resistance state that is 1000 times or more than that of a memory device. It is extremely important to suppress an excess current path from occurring.